Subject: Help

From: Monica Concha <[email-masked]>
Date: Tue, 16 May 2000 12:51:13 -0500

Hi all,

    I have been trying to optimize the vanadium dimer with multiplicity
of 3, using various dft methods. The problem is that depending on the
initial bond length, the dimer optimizes to different bond lengths with
different energies, and differ s**2 values.  The bond lengths only
differ
by about 0.01 Angstroms but the energies differ by 35 kcal.  The odd
thing is that the lowest energy corresponds to an s**2 value of 2.55
rather than the expected value of 2.0. Any suggestions?

--

Monica C. Concha
Research Associate
Dept. Of Chemistry
University of New Orleans
New Orleans LA 70148
E-mail [email-masked]

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From: "Jason K. Perry" <[email-masked]>
Date: Tue, 16 May 2000 16:03:35 -0700

Hi Monica,

    If you haven't done much work with metal clusters before, you're quickly
going to find out they are much tougher than simple organics.  The problem is
there are lots of low-lying excited states arising from low overlap of the
incomplete d shells.  In the case of V2, you have two dominantly s1d4 atoms
bonded to one another, with the other atomic states, d5 and s2d3, probably
mixing in.  This gives you a large number of possible spin and orbital
combinations to work with. 

    As for your DFT calculations, it's a given that you have to specify spin. 
But in these cases, you may also have to play with the orbital occupations in
the initial guess.  It's very easy for most codes to converge to excited
states of these molecules.  The only solid solution is to be very thorough. 
You've found two different triplet states already, but it might not be
guaranteed that either is actually the lowest energy triplet (or the ground
state for that matter!).

   Be aware of what states (d orbital occupations) are being converged.  Try
switching around the orbital occupations and converging some other states. 
Most codes offer some way of doing this either with the default initial guess
or with a restart file.

    If you're worried about spin contamination, you might want to try a
restricted open-shell approach.  I'd suggest sticking with unrestricted,
though.  In this case, I'd probably turn off any symmetry restrictions to
allow the spins to localize.  I haven't reviewed the literature on V2, but I'd
guess the ground state is singlet with one V alpha-spin s1d4 and the other
beta-spin s1d4.  The strong exchange interaction between the d orbitals should
be enough to localize the spin. But low-lying spin states could extend up to
nonet states.
    Good Luck,

    Jason Perry

--
..................................................................
Jason K. Perry, Ph.D.
First Principles Research, Inc.
8391 Beverly Blvd., #171, Los Angeles, CA 90048
phone (323) 465-9572
FAX (323) 465-2805
http://www.firstprinciples.com
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